1. Field of the Invention
The present invention relates to main spindle devices of machine tools.
2. Description of the Related Art
Main spindle devices of machine tools that machine a workpiece with a changeable rotary tool such as a machining center typically have a housing, a main spindle, a rod, and a piston. The main spindle is supported so as to be rotatable with respect to the housing. The rod is accommodated in the main spindle and is supported so as to be able to reciprocate in an axial direction of the main spindle. A collet that holds and releases a tool attached to the distal end (first end) of the main spindle is mounted on the distal end (first end) of the rod, and the rod is biased in a direction toward the rear end (second end) with respect to the main spindle by a plurality of disc springs. When the rod is biased by the disc springs and move to a clamp position in the direction toward the rear end, the rod pulls the tool being held by the collet in the direction toward the rear end, so that the tool is fixed to the distal end of the main spindle. The piston is disposed on the rear end side with respect to the rod so as to be slightly separated from the rear end of the rod. When the tool is changed or removed, the piston is used to move the rod in the direction toward the distal end by force larger than the biasing force of the disc springs. When the rod is moved to an unclamp position in the direction toward the distal end with respect to the main spindle, the collet releases the tool so that the tool can be changed or removed.
In such a main spindle device of a machine tool, the biasing force of the disc springs greatly affects the force supporting the tool. A decrease in biasing force of the disc springs due to their deterioration therefore greatly affects machining accuracy etc. If the operator machines workpieces without noticing such a decrease in biasing force of the disc springs, defective products may be produced as the workpieces are not machined with desired machining accuracy, or abnormality may occur during machining, stopping the machine tool for a long time. Accordingly, in production sites using such main spindle devices of machine tools, the biasing force of the disc springs is regularly checked and the disc springs are replaced before they reach the end of their life.
Checking the biasing force of disc springs is a very laborious task. Conventionally, a clamping force measuring gauge TK (see FIG. 17) or TR (see FIG. 18) that looks similar to a normal tool T shown in FIG. 16 is attached to the distal end of the main spindle instead of the tool T, and the biasing force of the disc springs is measured with the clamping force measuring gauge TK, TR. The clamping force measuring gauge TK shown in FIG. 17 has an accommodated portion TK1 that is to be accommodated in the main spindle and a protruding portion TK2 that is to protrude from the main spindle. The accommodated portion TK1 has substantially the same shape as that of the tool T. However, the protruding portion TK2 has a mechanical configuration and an electronic circuit for measuring the biasing force, a display unit, etc. The protruding portion TK2 therefore has a significantly different shape from that of the tool T and is much heavier than the tool T. The clamping force measuring gauge TR shown in FIG. 18 wirelessly transmits the measurement result etc. to a wireless unit TRC, and has an accommodated portion TR1 that is to be accommodated in the main spindle and a protruding portion TR2 that is to protrude from the main spindle. The accommodated portion TR1 has substantially the same shape as that of the tool T. However, the protruding portion TR2 has a mechanical configuration and an electronic circuit for measuring the biasing force, an electronic circuit for transmission, etc. The protruding portion TR2 therefore has a significantly different shape from that of the tool T and is much heavier than the tool T. Accordingly, when being attached to or detached from the main spindle by an automatic tool changer etc., the clamping force measuring gauge TK, TR may not be able to be appropriately attached to or detached from the main spindle. The expensive clamping force measuring gauge TK, TR may therefore drop and break.
Japanese Patent Application Publication No. 2008-246610 (JP 2008-246610 A) discloses a main spindle device of a machine tool in which a pressure sensor that detects a pressing force applied to press a rod to an unclamp position is attached to the distal end of a piston, and a rod displacement detector that detects displacement of the rod is provided in a housing. The main spindle device disclosed in JP 2008-246610 A presses the rod located at a clamp position in a direction toward the unclamp position by the piston and obtains the clamping force of a collet based on the pressing force of the piston and the displacement of the rod to determine if it is time to replace disc springs.
Ideal springs have linear characteristics having a linear relationship between the load and the deflection. However, actual springs (especially disc springs) do not have ideal linear characteristics, but have nonlinear characteristics with hysteresis as shown by load-deflection characteristics in FIG. 6. In the load-deflection characteristics shown in FIG. 6, a load Fa is a load corresponding to deflection Pc in the case where the load is gradually increased from zero, and a load Fb is a load corresponding to deflection PC in the case where the load is gradually reduced after being increased from zero to the load corresponding to deflection Pa. Due to the hysteresis, the load corresponding to the same deflection Pc (PC) varies depending on whether the deflection is increased from zero (no load) or the deflection is reduced from the large deflection Pa. In the load-deflection characteristics shown in FIG. 6, a path in which the load is gradually increased from zero to the load corresponding to the deflection Pa is referred to as the “forward path,” and a path in which the load is gradually reduced from the load corresponding to the deflection Pa to zero is referred to as the “return path.”
In the case of the main spindle devices of the machine tools, a load is applied to the disc springs by the piston until the rod reaches the unclamp position corresponding to the deflection Pa in the load-deflection characteristics shown in FIG. 6. The rod is then gradually moved in a direction toward the clamp position by the biasing force of the disc springs, and the tool is held clamped with the rod being located at the clamp position corresponding to the deflection PC. In order to obtain the biasing force of the disc springs in the main spindle devices of the machine tools, the load (load Fb) corresponding to the deflection PC in the return path of the load-deflection characteristics in FIG. 6 needs to be obtained rather than the load (load Fa) corresponding to the deflection Pc in the forward path thereof.
However, the main spindle device described in JP 2008-246610 A obtains the load (load Fa) corresponding to the deflection Pc in the forward path of the load-deflection characteristics shown in FIG. 6. The main spindle device described in JP 2008-246610 A therefore obtains the load larger than the load (load Fb) to be actually obtained. In this case, since the actual load (load Fb) of the disc springs when the tool is clamped is smaller than the obtained load (Fa), the time to replace the disc springs may have already passed, which is not preferable.